Update String.Contains (dotnet#4250)

Author: danmoseley

xml/System/String.xml

@@ -4105,7 +4105,13 @@ Examples of instantiating strings:
         <summary>Returns a value indicating whether a specified character occurs within this string.</summary>
         <returns>
           <see langword="true" /> if the <paramref name="value" /> parameter occurs within this string; otherwise, <see langword="false" />.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+ This method performs an ordinal (case-sensitive and culture-insensitive) comparison.
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Contains">
@@ -4160,7 +4166,7 @@ Examples of instantiating strings:
 ## Remarks
  This method performs an ordinal (case-sensitive and culture-insensitive) comparison. The search begins at the first character position of this string and continues through the last character position.
 
- To determine whether a string contains a specified substring by using something other than ordinal comparison (such as culture-sensitive comparison, or ordinal case-insensitive comparison), you can create a custom method. The following example illustrates one such approach. It defines a <xref:System.String> extension method  that includes a <xref:System.StringComparison> parameter and indicates whether a string contains a substring when using the specified form of string comparison.
+ **.NET Framework only**: To determine whether a string contains a specified substring by using something other than ordinal comparison (such as culture-sensitive comparison, or ordinal case-insensitive comparison), you can create a custom method. The following example illustrates one such approach. It defines a <xref:System.String> extension method  that includes a <xref:System.StringComparison> parameter and indicates whether a string contains a substring when using the specified form of string comparison.
 
  [!code-csharp[System.String.Contains#1](~/samples/snippets/csharp/VS_Snippets_CLR_System/System.String.Contains/cs/ContainsExt1.cs#1)]
  [!code-vb[System.String.Contains#1](~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.String.Contains/vb/ContainsExt1.vb#1)]